The invention relates to bicycle shifting systems, and more particularly, to a multi-speed sprocket assembly for a bicycle shifting system that ensures proper shifting from a smaller sprocket to a larger sprocket.
Derailleurs, in particular for bicycles, have existed for a long time. In spite of their open and therefore unprotected construction as compared, for example, to hub shifters, derailleurs are very popular due to their high level of efficiency, light weight, and cost-effective structure. In recent times, attempts have been made to improve shifting function, in particular for shifting under load as well as to reduce the gear step by correspondingly increasing the number of sprockets. For example, a ten-gear shifting system has been attempted with ten respective sprockets, with consecutive sprockets differing from one another by only one tooth.
In derailleurs, the most critical issue is shifting up from a smaller sprocket to a larger sprocket, under load if possible. In such cases, the provision of a special upshift transitional region in which favorable conditions exist for shifting has been known. No shifting occurs in the regions between the transitional regions because, in these regions, the chain is deflected from the larger gear wheel and remains on the smaller gear wheel.
In order to ensure an even and jolt-free shifting process in the transitional regions, the same gear wheels are offset from one another at an angle such that, taking into account the chain pitch, the chain assumes a desired course. However, it must be kept in mind here that the chain does not include identical chain links; rather, it includes alternating chain links with interior link plate pairs, the inner links, and chain links with outer link plate pairs, the outer links. Therefore, when a leading tooth of the larger sprocket cooperates for the first time with a chain link while the chain is being thrown, it often occurs that, if this leading chain link is an outer link, then the leading tooth engages between the outer link plates, but when the leading chain link is an inner chain link, then the leading tooth is located next to the inner pair of link plates. This situation must be taken into account when designing the derailleur. Here, it is important in both shifter configurations, among other things, to ensure that the teeth following the leading tooth opposite the running direction be securely engaged in the chain. Thus, the chain must be prevented from “riding” on the larger sprocket, in that a chain link lands on a tooth tip.
To ensure proper shifting, sufficient room must be provided for the oblique chain, which is normally achieved by providing corresponding recesses on the side of the larger sprocket facing smaller sprocket. Here, the base of the recess forms a contact surface for a chain link or a chain pin of the chain that is crossing from the smaller sprocket to the larger sprocket for the axial alignment of the chain (EP 0 313 345 B1).
A radial guide of a link pair may be provided in the transitional region, specifically with the aid of a link contact ramp on the side of the larger sprocket facing the smaller sprocket in an end region of a double spacewidth of the large sprocket (EP 0 642 972). Because an axial guide is present as well, there are good conditions per se for a flawless shifting function. However, it has been shown that, under certain conditions, the chain slides off of the ramp and is displaced in the direction of the small sprocket. This leads to an extension of the crossing chain section, which is comprised of two chain links, with the result of faulty shifting function.
The above solutions are sensitive to dirt because dirt between the radial or lateral guide surface and the link plate may lead to a malfunction. In general, it is also possible to use only one type of chain, which is adapted to the multi-speed sprocket assembly. Although the shifting chains produced by different manufacturers have the same basic dimensions (interval 12.7 mm, inner width 2.38 mm, and roller diameter 7.75 mm in accordance with ISO 082 for derailleurs), there are differences in the structure of the outer link plates, for example, due to an arcing of the outer link plates, as well as in the length of the link pins with a corresponding larger or smaller distance past the respective outer link plate.
DE 39 36 921 A1 discloses a multi-speed sprocket assembly with a tooth number difference of 1 between consecutive sprockets. In order to facilitate the crossing of the chain and thus obtain a radial guide, a tooth of the smaller sprocket is shaped in the transitional region to correspond with the contour of the crossing link plate, in order to be able to radially support, in places, the crossing link plate that is located farther from the sprocket.